Substrates used for the production of thick film and integrated circuits may be produced by the application of a layer of vitreous or porcelain enamel to a base, which is generally metallic and usually steel. The vitreous enamel is usually applied to the base by dipping, spraying or electrophoretic deposition (note: other methods are sometimes employed) and is then fired to cause fusion or sintering of the enamel into a coating adherent to the base. The enamel when applied to the base may have all or some of its constituents premixed, presintered or premelted, or a mixture of the enamel constituent may be applied.
These existing methods suffer from disadvantages. It is difficult to achieve flatness when enamel powder is applied to the base and this lack of flatness is reproduced to a lesser or greater extent when the coating has been fired. This lack of flatness causes problems when circuitry is applied to the surface, typically but not exclusively by the process known as silk screen printing wherein the screen material is usually synthetic fibre or metal. Also, the techniques at present in use frequently allow air bubbles to exist in the final fired coating, leading to a reduction in the electrical resistance value. Also, as progressively thicker coatings are applied, the problems of unevenness of coating and air bubbles increase so that the maximum thickness of coating which can be applied in practice may be less than desirable for certain applications.
There is also a tendency for iron ions from the substrate to migrate into the enamel surface causing electrical problems, and such migration is usually precluded or minimised by applying a barrier layer of some sort, usually nickel, to the surface of the steel.
Holes through metal bases which are to be vitreous enamelled present a problem in that severe thinning of the enamel occurs at the sharp edges which remain when holes are produced, as by punching or drilling. This results in low electrical resistance or, in extreme cases, short circuiting between the metal base and conductor wires passing through such holes. To minimise such problems it has been proposed that the edges of the holes be rounded or coined so that the tendency for the enamel to thin or draw away is minimised. This approach is not always satisfactory and in any case increases the cost of the preparation of the substrates.